Standardise messages about missing modules to include problem, consequence and what to do about it (currently including references to GEPS 029 where necessary).

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21671 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
Revert change to const.py

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21673 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
README file. Add PyICU as strongly recommended.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21675 4ae1f11a-8b86-4847-b8af-ab372f36d1fd